1. Field of the Invention
The present invention relates to a trans-conductance amplifier, and more particularly to, a digitally programmable trans-conductance amplifier and mixed signal circuits using the same.
2. Description of the Prior Art
The conventional current mirror for the trans-conductance amplifier is shown in FIG. 1. The programmable current mirror disclosed in the prior art is programmed by changing the width of the parallel transistors at the output stage of the current mirror so as to change the output current. However, said programmable current mirror, if being applied to the output stage, cannot address some issues such as gain, bandwidth, stabilities, because the output current size change will have direct effect on its output impedance and the same is an important variable related to bandwidth and stabilities.
As suggested in U.S. Pat. Nos. 6,466,100 B2 and 6,462,527 B1, in both of the prior arts digitally control signals are used to control and switch the counting of parallel transistors so as to change the equivalent widths of the transistors and a variable current source is used to provide the bias current of the output stage. The drawbacks of these two prior arts are that the variation of the bias current of the output stage according to the control signal is significant and thus their output impedance varies significantly.
Also, in U.S. Pat. No. 6,456,158 B1 suggested a trans-conductance amplifier, which is digitally programmable, however, a plurality of resistors invite noises and at output stage, inevitably, there needs a current follower to enhance the output stage to be high impedance.
Hence, the present invention provides a solution for a trans-conductance amplifier, particularly a digitally programmable trans-conductance amplifier, to address the issue of the conventional programmable current mirrors. The present invention is assumed that the output current is a constant and the switch can be turned on or off to change the reference current and the widths of the transistors so as to achieve the goal for programmable current mirrors. The disclosures of the present invention suggest reforming all current mirrors to be programmable and suitable for the second stage of the trans-conductance amplifier which reforms the trans-conductance amplifier with fixed gain to be of variable trans-conductance so as to let the mixed mode signal circuit using the same with variable gain can base on the feedback loop to achieve the goal of setting the corresponding parameters to be constant.